An important use of radiotherapy, and in particular intensity-modulated radiation therapy (IMRT), is the destruction of tumor cells. In the case of ionizing radiation, tumor destruction depends on the “absorbed dose”, i.e., the amount of energy deposited within a tissue mass. Radiation physicists normally express the absorbed dose in cGy units or centigray. One cGy equals 0.01 J/kg.
Radiation dosimetry generally describes methods to measure or predict the absorbed dose in various tissues of a patient undergoing radiotherapy. Accuracy in predicting and measuring absorbed dose is key to effective treatment and prevention of complications due to over or under exposure to radiation. Many methods exist for measuring and predicting absorbed dose, but most rely on developing a calibration—a curve, lookup table, equation, etc.—that relates the response of a detection medium to the absorbed dose. Useful detection media are known to those skilled in the art and include radiation-sensitive films and three-dimensional gels (e.g., ‘BANG’ and ‘BANANA’ gels) which darken or change color upon exposure to radiation. Other useful detection media include electronic portal-imaging devices, Computed Radiography (CR) devices, Digital Radiography (DR) devices, and amorphous silicon detectors, which generate a signal in response to radiation exposure.
There are various known methods for developing a calibration curve. For example, U.S. Pat. No. 6,675,116, assigned to the assignee of the present application and fully incorporated herein by reference, discloses providing a detection medium that responds to exposure to ionizing radiation, and preparing a calibration dose response pattern by exposing predefined regions of the detection medium to different ionizing radiation dose levels. The '116 patent further discloses measuring responses of the detection medium in the predefined regions to generate a calibration that relates subsequent responses to ionizing radiation dose. Different dose levels are obtained by differentially shielding portions of the detection medium from the ionizing radiation using, for example, a multi-leaf collimator, a secondary collimator, or an attenuation block. Different dose levels can also be obtained by moving the detection medium between exposures. The '116 patent further discloses a software routine fixed on a computer-readable medium that is configured to generate a calibration that relates a response of a detection medium to an ionizing radiation dose.
Methods such as those disclosed in the '116 patent require exposing discrete portions of the detection medium to different and known amounts of radiation using a linear accelerator or similar apparatus in order to develop a calibration curve or lookup table. Typically about twelve, but often as many as twenty-five, different radiation dose levels are measured in order to generate a calibration curve or look-up table. Generally, the accuracy of the calibration increases as the number of measured radiation dose levels increases. However, the greater the number of measurements, the more expensive and time consuming the calibration process becomes. Thus, it would be desirable to have a system and method that provides calibration information by analyzing one “acquired image” obtained by applying a radiation therapy plan to a quality assurance device and capturing the radiation intensity distribution.
Methods are known for correcting an acquired image so that a dosimetry acquisition system that has been once calibrated will not have to be recalibrated each time. For example, U.S. Pat. No. 6,528,803, assigned to the assignee of the present application and fully incorporated herein by reference, teaches exposing portions of test films to an array of standard light sources to obtain an optical density step gradient, which can then be compared to a corresponding optical density step gradient on one or all of a set of calibration films. However, existing methods such as those disclosed by the '803 patent require additional equipment and time to gather data relating to the optical density step gradient. In some cases, it would be desirable to have a system and method providing calibration information for a subsequent acquired image that took a minimum amount of time even if this was only a “relative” calibration (expressed in percent). In other cases, it would be desirable to have a system and method providing “absolute” calibration information (e.g. based on a national standard) for use in generating a “relative” calibration.
Further, it may also be desirable to have a system to evaluate the ability of experimentally derived calibration curves to model “absolute” dose distributions produced by application of treatment plans in order to determine where differences occur by modeling inaccuracies as opposed to “absolute” measured responses.